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Welcome Rains Are Forecast, But So Are Severe Storms

Published: October 12th, 2012, Last Updated: October 12th, 2012

When Midwestern farmers pray for rain during this dismal drought year, they probably aren't hoping that it would come in the form of severe thunderstorms, which carry the risk of crop-killing hail and tornadoes. Yet that is what they are likely to get during the next few days, as a strong storm system pivots out of the Southwest and heads into the Plains and Midwest.

Projection for the amount of precipitation that may fall during the next five days, showing the swath of heavy rains from Texas to Wisconsin.Click to enlarge the image. Credit: NOAA/HPC.

The National Weather Service summarized the situation in a Facebook post, stating: "The NWS Storm Prediction Center is forecasting a risk of severe weather Friday afternoon into the overnight hours that will be a prelude to what could be one of the most active weather weekends of 2012."

Despite the concerns about severe weather, the storm will at least help lessen the drought, which is one of the worst to strike the U.S. since the 1930s “Dust Bowl” era.

The severe weather is expected to be triggered by a weather system at the upper levels of the atmosphere, known as an upper level low, that is forecast to move out of the Southwest on Friday, emerging in the Plains and eventually the Midwest over the weekend. This weather system brought severe thunderstorms with hail and flash flooding to the Los Angeles and San Francisco areas on Thursday.

The rains will fall in some of the areas that have been hardest hit by the drought.Click to enlarge the image. Credit: NOAA/USDA.

The storm will draw warm and humid air northward from the Gulf of Mexico, helping to generate an elongated area of showers and thunderstorms from Texas northward to Minnesota.

Severe thunderstorms with hail, damaging winds, and tornadoes are possible along a cold front that will move east with time, particularly during the afternoon on Saturday and Sunday. Cities such as Dallas, Oklahoma City, Kansas City, Chicago, and Minneapolis-St. Paul are at risk for these storms.

Tornadoes can occur during any month of the year in the U.S., and so far this year, there have been fewer tornadoes than average. Through Sept. 21, the U.S. had seen 721 tornadoes, which is well below average.

That stands in stark contrast to 2011, in which 1,692 tornadoes killed hundreds of people during several severe weather outbreaks of unusual scope and ferocity.

The same weather pattern that contributed to the severe drought has squelched severe thunderstorms and the tornadoes they can spawn, particularly across the so-called “Tornado Alley” in the Great Plains and Midwest. During July, for example, Canada saw more tornadoes than the U.S. did, which is a rare occurrence.

The storm system will draw warm and humid air northward from the Gulf of Mexico, as seen in this computer model forecast of atmospheric moisture (known as precipitable water).Click to enlarge the image. Credit: Weatherbell.

Although it may spark damaging thunderstorms, drought relief is the bright side to the upcoming storm system. It is expected to provide beneficial rainfall, on the order of 1 to 2 inches, in some of the hardest hit states. Computer-model projections show the likelihood for particularly heavy rains in parts of Oklahoma. As of Oct. 2, about 81 percent of the state was experiencing “extreme” to “exceptional” drought conditions.

Beneficial rainfall is also projected to fall in other hard-hit states, such as Iowa, Illinois, Minnesota, Missouri, and Wisconsin.

While 2 inches of rain would certainly help the drought situation, much of the drought region needs upwards of a foot of rain to put an end to the drought. It will take repeated storms in order to accomplish that, and climate outlooks for the next three months are ambiguous about those prospects.

The newest edition of the U.S. Drought Outlook, which was released on Oct. 4, shows that the drought is expected to hang on in the West and Upper Midwest, but improving conditions are likely in some of the areas that are going to be affected by this latest storm system.